Ultrasound imaging provides a real-time image (e.g., a B-mode image) with structural information about the interior of a subject such as organs and blood vessels. Ultrasound imaging has additionally been used to concurrently visualize flow inside the blood vessels. Color Flow Mapping (CFM) imaging is one such approach. With CFM imaging, a number of pulses are directed to each line-of-sight in a color box, and the returning echoes are used to determine flow direction (towards or away from the transducer) in the color box. A color flow map with colors that indicate flow direction is generated and overlaid over the B-mode image to show flow direction relative to the underlying vessel, etc. structure in the B-mode image.
CFM imaging, relative to B-mode imaging, consumes the majority of each frame, even with a color box that is significantly smaller than the B-mode image. The transducer elements are switched between B-mode and CFM imaging for respective imaging. The pulses transmitted for CFM imaging (e.g., 10-16 each frame) along each color scan line of the image give a frequency shift at each area of measurement, and this frequency shift is displayed as a color pixel. The scanner repeats this for several lines to build up the color image, which is superimposed over the B-mode image. CFM imaging may have to produce many color points of flow information for each frame.
For CFM imaging, the pulse-repetition frequency (PRF) determines a maximum velocity that can be unambiguously measured. That is, where the flow being measured is greater than the half of the PRF, aliasing occurs, and flow moving towards the transducer may be interpreted by the system and displayed as flow moving away from the transducer, and vice versa. Unfortunately, as framerate as well as color sample density is clinically important, it is not always possible to acquire color information at a PRF that allows for unambiguous separation of flow moving towards the transducer from flow going away from the transducer.
Slower flow (e.g., diastole flow, flow close to vessel walls, etc.) may not be identified using a higher PRF. As such, a lower PRF can be employed to measure slower flow even though there will be aliasing in other vessels with faster flow. In this instance, to reduce confusing the user, color maps are created so that the transition from the most positive velocity to the most negative velocity occurs for color that appear approximately equally bright to the user but have different hues. Unfortunately, with strong aliasing, it becomes difficult to look at these images, which result in the user setting a lower limit for the PRF.
There is also a coupling to the fidelity of the acquired color flow images. The raw color flow images are often very noisy and are often of low spatial resolution so that they require significant image processing in order to appear aesthetically pleasing and provide the user diagnostic confidence. As most image processing algorithms such as smoothing and upsampling algorithms are not designed for aliasing, image processing of even mildly aliased images creates significant artifacts in the processed color flow images and these artifacts make users question the diagnostic confidence in the imaging information and therefore avoid lower PRFs even though these images correctly interpreted provide additional useful clinical information.